Natural rubber has been widely employed in industrial articles such as automobile tires, belts and adhesives and domestic articles such as gloves.
In addition to its excellent mechanical properties as vulcanized rubber, natural rubber is much superior in the raw rubber strength (green strength) to synthetic rubber. Accordingly, natural rubber is excellent in processing characteristics in kneading, sheeting and various molding procedures. In the form of a latex, natural rubber has a high gel strength at solidification and thus can be easily formulated into a film, which makes the natural rubber applicable to various products including condoms, surgical gloves and diverse catheters.
This natural rubber is obtained as Hevea tree sap which is in the form of a latex containing rubber components together with water, proteins and inorganic salts. A green latex obtained by tapping Hevea trees is collected in cups and combined together. Then it is solidified in a refining plant to thereby give a raw rubber (i.e, a crepe rubber or a smoked sheet rubber). Alternately, the green latex is concentrated by centrifuging to thereby give a refined latex. During these processes, ammonia is usually added to the latex in order to inhibit the progress of putrefaction due to the bacteria contained in the green latex and to prevent the latex from solidification. In recent years, attempts have been made to prevent the putrefaction by previously adding a small amount of zinc oxide and tetramethylthiuram disulfide to the cups into which the green latex is to be collected. In each rubber farm, the green latex collected in the cups is combined and a small amount of ammonia is further added thereto, followed by transporting to a refining plant. In the refining plant, the green latex is diluted with water, solidified by using formic acid and dried into raw rubber. Alternately, the green latex is adjusted to an ammonia concentration of 0.5%, separated from the latex serum by centrifuging, concentrated and formulated into a high-ammonia latex of an ammonia concentration of 0.7% so as to prevent the putrefaction and to mechanically stabilize or to a low-ammonia latex of an ammonia concentration of 0.2% and containing a small amount of tetramethylthiuram disulfide, followed by shipping.
The rubber components in the natural rubber latex are solidified to thereby give a raw rubber (a crepe rubber or a smoked sheet rubber) which is then subjected to mastication, blending with additives, molding and vulcanization to thereby give the aimed rubber products.
Mastication means a treatment whereby a shear force is applied to the latex to thereby loosen molecular aggregates and cleave molecular chains, thus reducing the molecular weight. In order to accelerate the occurrence of these reactions, loosening agents are sometimes used. Thus the elasticity of the rubber is lowered and the rubber becomes plastic, which makes the processing operations following the blending step easy.
In general, the plasticity of unvulcanized rubber is expressed in Mooney viscosity. A higher Mooney viscosity means the higher elasticity and the lower plasticity of the rubber. On the contrary, a rubber of a low Mooney viscosity has a low elasticity and a high fluidity, which makes it easy to process the rubber. In practice, a high Mooney viscosity results in a large extrusion torque in, for example, the subsequent extrusion step. As a result, the extrusion speed decrease, which lowers the production efficiency. In addition, the large extrusion torque causes the accumulation of the energy in the rubber. Consequently, the rubber becomes hot and, in its turn, undergoes scorching. It has been known that the addition of an oil would lower the Mooney viscosity of rubber. However, it is impossible in some cases to use any oil. Accordingly, there has been required to provide a natural rubber of excellent processing characteristics.
On the other hand, it has been recently reported in United States that medical instruments with the use of natural rubber such as surgical gloves, various catheters and anesthetic masks cause dyspnoea or anaphylactoid symptoms (for example, angioedema, urticaria, collapse, cyanosis). It has been also reported that female patients with allergic anamnesis suffered from hand ache, urticaria and angioedema around eyes due to the use of household natural rubber gloves.
Accordingly, it has been required to develop measures against such problems.
The reduction in the content of non-rubber components in natural rubber contributes to the lowering of the water absorptivity of the natural rubber and to the improvement in the electrical characteristics (for example, electrical insulation properties) of natural rubber products. By almost completely eliminating non-rubber components, an advantageous material for producing rubber products, which suffer from little energy loss and have excellent mechanical properties, improved crepe characteristics and improved aging resistance, can be provided. It is one of serious disadvantages of natural rubber that the material properties vary depending on the production area and production time, which is a problem characteristic of natural products. Therefore, the elimination of the non-rubber components causing this problem makes the vulcanizing characteristics stable. Thus the natural rubber becomes a material rubber having stable qualities comparable to synthetic rubbers. Thus the mechanical accuracies of natural rubber products can be elevated.
Known methods for reducing the non-rubber component content in natural rubber comprise thoroughly washing a latex with water. That is to say, there have been known methods therefor which comprise (i) aggregating rubber particles in a highly diluted latex; (ii) centrifuging a highly diluted latex and separating the latex thus concentrated; or (iii) dialyzing a latex.
Further, there have been known other methods for reducing the non-rubber component content in natural rubber which comprise (a) decomposing the non-rubber components with bacteria or enzymes; (b) adding alkali to a latex and heating the obtained mixture to thereby decompose the non-rubber components; or (c) liberating non-rubber components adsorbed by rubber particles with the use of soaps.
In practice, deproteinized natural rubbers are produced by combining some of these methods with each other. Examples of such deproteinized natural rubbers are as follows.
(1) Crepe H:
A small amount of ammonia is added to a latex to thereby make the pH value of the latex 7.1, followed by stirring for 6 to 48 hours. During the stirring procedure, proteins are decomposed by bacteria and enzymes contaminating the latex after collecting the sap. In this case, the solidified product which is preliminarily formed is removed and then the decomposition product is eliminated by centrifuging or creaming. Next, the solidified product is formulated into a crepe.
(2) Crepe G:
To a latex stabilized with ammonia, a soap or other surfactant(s) are added to thereby adsorb proteins. Then salts and proteins are eliminated from the latex by repeatedly centrifuging. Next, the latex is highly diluted and solidified to thereby give a crepe.
(3) Crepe CD:
In this case, a fresh-solidified product before rolling is immersed in running water and thus proteins are decomposed. After separating by dialysis, the product is formulated into a crepe.
On the other hand, there has been proposed an improved deproteinized natural rubber which is produced by lowering the ammonia concentration of a concentrated latex preserved in the presence of ammonia to 0.2%, adding 0.4 phr (parts per hundred) of ammonium naphthenate as a preservative, carrying out the enzyme reaction for 20 hours by adding 0.25 phr of superase (a protease), then diluting the latex and solidifying the same with phosphoric acid, as disclosed in Tennen Gomu (natural rubber), vol. 6, No. 8, 274-281 (1974).
The protein content of a natural rubber is generally expressed in an amount corresponding to 6.3 times as much as its nitrogen content (N %) which is determined by Kjeldahl method. The protein content of a fresh natural rubber latex (field latex) ranges from about 3 to 5% by weight (about 0.5 to 0.8 as N %) based on solid matters (rubber particles). The protein contents of a marketed refined latex and raw rubber (smoked sheet rubber) are about 2% by weight (about 0.3 as N %) or above. Although the protein contents of the above-mentioned deproteinized natural rubbers are remarkably lowered compared with these marketed natural rubbers, the N % of the crepe CD is 0.11 while that of the deproteinized natural rubber obtained by the latter improved method is 0.06. Thus none of them is completely deproteinized and, therefore, satisfactory as a material for solving the above problems.